1. Field of Invention
The present invention relates to a power supply circuit for feeding power with a small loss, and an electro-optical device to which power is fed using the power supply circuit and which is characterized by low power consumption.
2. Description of Related Art
To begin with, a description will be made of a conventional power supply circuit, for example, a power supply circuit for producing a high voltage Vout from a low voltage Vin and supplying the high voltage Vout. FIG. 9 is a block diagram showing the circuitry of a conventional power supply circuit 400. In the drawing, a switch 412 is realized with a transistor in practice, and turned on when a signal CTR that is a gate signal is driven high. One terminal of the switch 412 is connected on a line, over which the voltage Vin is applied, via an inductor (coil) 414. The other terminal thereof is connected on a ground line over which a reference potential GND is applied. One terminal of the switch 412 is connected to one terminal of a capacitor 418 through the conduction in a forward direction of a diode 416. Consequently, a held voltage is output as the voltage Vout The other terminal of the capacitor 418 is connected on the ground line.
The output voltage Vout developed at one terminal of the capacitor 418 has a fraction thereof developed due to resistors 420 and 422. The fractional voltage is applied to a negative input terminal of a comparator 424. For a clear description, a voltage to be applied to the negative input terminal of the comparator 424 shall be a voltage Vout'. A reference voltage Vref is applied to a positive input terminal of the comparator 424. When the voltage Vout' falls below the reference voltage Vref, an output signal Cow of the comparator 424 is, as shown in FIG. 10, driven high. In contrast, when the voltage Vout' exceeds the reference voltage vref, the output signal CMP is driven low. When the output signal CMP of the comparator 424 is driven high, a control circuit 426 outputs, as shown in FIG. 10, a pulsating signal CTR having a certain pulse width W.
Next, the action of the power supply circuit 400 having the foregoing components will be described. First, the switch 412 is turned on. This causes a current Ion to flow from the line, over which the voltage Vin is applied, through the inductor 414 towards the ground line. Energy is accumulated in the inductor. When the switch 412 is turned off, an off current loff flows through the inductor 414. The accumulated energy is added to the voltage Vin through the conduction in the forward direction of the diode 416 according to the series feed. The energy thus moves to the capacitor 418. When the energy accumulated in the inductor 414 has entirely moved to the capacitor 418, the diode 416 is inversely biased. Therefore, the energy accumulated in the capacitor 418 will not flow backward through the diode 416.
On the other hand, the output voltage Vout drops gradually according to the magnitude of a load. As shown in FIG. 10, when the voltage Vout' falls below the Ad reference voltage Vref, the output signal CMP of the comparator 424 makes a low-to-high transition. This causes the control circuit 426 to output the pulsating signal CTR. The switch 412 is then turned on. After energy is accumulated in the inductor 414, the switch 412 is turned off, and the energy moves to the capacitor 418. Consequently, the output voltage Vout rises. In other words, when the voltage Vout' falls below the reference voltage Vref, control is given to raise the output voltage Vout.
When the output voltage Vout rises, the voltage Vout' may exceed the reference voltage Vref. In this case, the output signal CMP of the comparator 424 remains low. The switch 412 is not therefore turned on or off, and the capacitor 418 discharges according to a load. Consequently, the output voltage Vout drops gradually. In other words, when the voltage Vout' exceeds the reference voltage Vref, the discharge of the capacitor 418 causes control to be given for lowering the output voltage Vout.
As a whole, the output voltage Vout is stabilized when control given for raising the output voltage Vout and a control given for lowering it are balanced, that is, when the voltage Vout' becomes equal to the reference voltage Vref. Herein, the voltage Vout' is a fraction of the output voltage Vout developed due to the resistors 420 and 422. Assuming that the resistances of the resistors 420 and 422 are R1 and R2, Vout'=Vout.times.R2/(R1+R2) is established. The voltage Vout' is stabilized when becoming equal to the reference voltage Vref In other words, the power supply circuit 400 boosts the voltage Vin and outputs the voltage Vout stabilized when becoming equal to Vref(R1+R2)/R2.